1. Technical Field of the Invention
The present invention relates to a power transmission device having a torque limiting function. More particularly, the present invention relates to a power transmission device having a torque limiting function preferably used when it is incorporated in a constant-operation type compressor, of an air conditioner for vehicle use, driven by an external power source such as an engine via a belt and others.
2. Description of the Related Art
A refrigerant compressor used for an air conditioner for vehicle use is driven by an external power source such as an engine via a belt, a pulley and others. In this case, in order to disconnect the refrigerant compressor from the engine, an electromagnetic clutch may be interposed between them. However, when the electromagnetic clutch is not incorporated between them, it is possible to reduce the manufacturing cost. Therefore, the electromagnetic clutch is omitted in many cases. In this case, in a power transmission device of the compressor, used for the air conditioner for vehicle use, driven by the external power source such as an engine, a torque limiter is provided to avoid the occurrence of a problem such as cutting of a belt when the compressor has seized.
For example, according to the official gazette of JP-A-2003-206950, a portion of a power transmission is engaged by means of screwing.
In this torque limiter system in which an engagement of screwing is used, by utilizing an excessively high axial tension generated in the screw engaging portion by an excessively high torque when a compressor has seized, a portion of a power transmission path is broken, so that the power transmission path can be cut off.
As this torque limiter system utilizes a screw engagement, the accuracy of operation of the limiter is affected by an intensity of torque generated at the time of seizing and by a coefficient of friction of the screw portions which are an engagement portion and seating faces.
Especially, according to the prior art shown in FIG. 5A in which a flat washer is used for the washer 8 coming into contact with the rotary shaft 4 of a compressor, for example, a seating face of the flat washer 8 coming into contact with the rotary shaft 4 is corroded with age when water enters from the outside and attaches to the seating face of the flat washer 8. In this case, the coefficient of friction is changed due to the corrosion. Accordingly, an intensity of torque of the torque limiter is changed, which deteriorates the accuracy of the torque limiter.
FIG. 20 is a sectional side view showing an example of the power transmission device of the prior art into which the above torque limiter is incorporated. FIG. 21 is a partially enlarged view of FIG. 20. FIG. 22 is a partial sectional side view showing a state in which the torque limiter of the example of the prior art is operated.
As shown in FIGS. 20 to 22, in the power transmission of a comparative example, the power shutoff member (the torque limiter) 3 is connected to the rotary shaft 4 of a compressor (not shown) by the screw portion 33 being screwed. Further, the power shutoff member (the torque limiter) 3 is connected to the inner hub 21 of the hub 2 provided on the pulley side by means of spigot engagement. When an axial shift is caused between the power shutoff member 3 and the inner hub 21, as shown in FIG. 21, a gap d is generated between the engagement portion 35 of the power shutoff member 3 and the engagement portion 21a of the inner hub 21. Therefore, the chamfered portion 21b of the hub 2 interferes with the root-rounded portion 36 of the power shutoff member 3, which affects a torque limiting characteristic of the power shutoff member 3. That is, a breaking force, which is determined by an axial tension acting on a sectional area of the breaking portion 34 of the power shutoff member 3 and substantially proportional to the given torque, is affected by a resistance force generated by the interference of the chamfered shape 21b with the root-rounded portion 36. Further, as can be understood from FIG. 22, unless a caulked portion is provided in the large outer diameter portion 31 of the power shutoff member 3, the large outer diameter portion 31 falls off at the time of operation of the torque limiter.
This torque limiter system is operated in such a manner that the power shutoff member is cut apart by an excessively high axial tension, which is generated by screw engagement, by utilizing an excessively high intensity of torque generated in the phenomenon of seizing of a compressor. In this case, the following problems may be encountered. An axis of the power shutoff member is shifted from an axis of the inner hub accommodating the power shutoff member. Therefore, it is impossible to obtain a stable power shutoff characteristic. At the time of operation of the power shutoff member, a breaking portion provided between the screw portion side and the large outer diameter portion is cut apart by an axial tension. After the completion of operation of the power shutoff member, the large outer diameter portion falls off.
In order to solve the above problems, a method is provided in which the large outer diameter portion, which is a portion of the power shut-off member, is press-fitted and fixed into the inner hub so as to prevent the occurrence of an axial shift and the large outer diameter portion is fixed by the press-fitted portion to prevent a disconnection of the large outer diameter portion at the time of operation of the torque limiter of the power shutoff member. However, when this method is adopted, stress is generated by press-fitting in a portion which affects the limiter operation characteristic of the power shutoff member, which has a poor influence on the power shutoff characteristic. Therefore, it is impossible to solve the above problems.